A method for controlling the power factor or the voltage in a three-phase network by a high-voltage d-c short coupling is already known from German Published Unexamined Patent Application No. 19 62 042. It is a method for controlling the active and reactive power transmission between two electrical networks which are coupled via a high-voltage d-c transmission line (HVDC line), where each network is coupled via a controlled converter to the HVDC line. One of the converters is operated as a rectifier and the other converter as an inverter. A first control angle control variable for a control unit of the converter operated as a rectifier is derived by a subordinated current controller for d-c current in the HVDC line, and a reference value for the current controller is derived by a superimposed power controller for active power in one of said networks. A second control angle control variable for the control unit of the converter operated as an inverter is derived by a superimposed controller for at least one of the voltage and the reactive power in one of said networks.
In addition to the active power behavior, the reactive load behavior is influenced in this known method by a suitable COntrol of the converter installation of the HVDC line. For this purpose, an adjustment of the d-c current and the d-c voltage on the HVDC line according to predetermined functions is carried out simultaneously. For instance, for operation with a constant power factor, the reference value for the current as well as for the voltage control are set in separately depending on the actual active power reference value by a function generator.
A known control structure is shown in FIG. 1, with its operation being explained below and with the aid of FIGS. 2 and 3.
In FIG. 1, a known control structure for a high-voltage d-c transmission line (HVDC line) is shown. Two or three-phase power supply networks A and B are coupled to the ends of an HVDC line via controlled converters UR1 and UR2. The electrical parameters of the line have the reference symbols Id for the d-c current and Ud for the d-c voltage in FIG. 1. A smoothing inductance Ld for damping harmonics in the HVDC line is shown. Depending on the desired direction of the power transmission over the line, one of the two converters UR1, UR2 operates as a rectifier and the other one as an inverter In FIG. 1, it is assumed that the converter UR1 works as a rectifier and the converter UR2 as an inverter.
Two cascade controls control the active power P in one of the two networks and either the network voltage U or the reactive power Q in the remaining network. These cascade controls act on one of the two converters and are shown in FIG. 1 on the left side of the vertical dashed line marked with II. To the right of the dashed line marked with I is the respective subordinated controls of the two cascade controls. The superimposed controls of the cascade controls are to the left of dashed line I.
The control deviation .DELTA. P formed from an active power reference P* and an active power actual value P is fed from summing junction 1 to a superimposed active power controller Rp. The output signal of this controller Rp serves as the reference value Id* for the d-c current on the HVDC line. The corresponding control deviation .DELTA. Id is formed at a second summing junction 2 by comparison of the reference value Id* with the actual d-c value Id. The actual control angle .alpha..sub.GR for the converter operated as a rectifier is formed by a subordinated d-c component Ri in FIG. 1. The actual control angle .alpha..sub.GR is converted in a first control unit STI into corresponding firing pulses for the semiconductor switching elements of the converter UR1.
The second cascade control acts on the converter UR2 operated as an inverter in the example of FIG. 1. A superimposed controller Rq serves here either as a controller for the network voltage U or as a controller for the reactive power Q in one of the two networks, depending on the arrangement. Thus, the voltage or reactive power control deviation .DELTA. U/ .DELTA. Q for this controller is formed by comparison of the corresponding reference value (U*/Q*) with the corresponding actual values U/Q at a third summing junction 3. In FIG. 1, the output signal of the superimposed controller Rq is the reference value .gamma.* for the quenching angle of the inverter UR2. A control deviation .DELTA..gamma. is formed by comparison of the reference value .gamma.* with the actual quenching angle value .gamma. at a fourth summing junction 4 and then fed to a subordinated quenching angle controller Rl. The control angle .alpha..sub.WR for the inverter UR2 is produced by controller Rl and then converted in a further control unit St2 into corresponding firing pulses for the semiconductor switching elements. The inverter influences, according to the actual value of the control angle .alpha..sub.WR, the d-c voltage Ud on the HVDC line as to magnitude and sign.
In a known embodiment, not illustrated, the subordinated quenching angle control Rl is omitted. In this case, the superimposed voltage or reactive power controller Rq sets the control angle for the inverter UR2 directly. In another known embodiment, the lead angle .beta. or the quenching angle .gamma. are used directly as a control input for the converter UR2 instead of the control angle .alpha..sub.WR which also takes into consideration the overlap angle u.
The operation of the known circuit of FIG. 1 will further be explained briefly with reference to the family of characteristics of FIG. 2. For this example, it is assumed that the superimposed controller Rq of the second cascade control influences the reactive power Q in one of the networks. Accordingly, the family of characteristics of FIG. 2 shows the relationship between the active power P and the reactive power Q, divided by the nominal values P.sub.N and Q.sub.N, respectively. As further parameters, the family of characteristics contains the d-c current Id on the HVDC line, the control angle .alpha..sub.WR during the rectified operation of the respective converter and the quenching angle .gamma..sub.WR in inverter operation of the respective converter.
Assuming that the converters URl and UR2 of the HVDC line of FIG. 1 are not equipped with additional reactive power generators which can be connected and disconnected, such as capacitor banks, smoothing chokes and transformer stepping switches, the family of characteristics in FIG. 2 shows all permissible operating points regardless of whether the converter in question is operated as a rectifier or an inverter. The portion of the family of characteristics shown in FIG. 2 is chosen so that the operating points which occur at small and medium values of the rectifier control angle or the inverter quenching angle up to about 60. el can be seen. If the influence of overlap, depending on the operating point, on the individual parameters of the family of characteristics is neglected, the straight line characteristics resulting for a constant rectifier control angle or inverter quenching angle go through the origin, not shown, of the diagram. In such a case, the rectifier control angle or the inverter quenching angle would be identical with the load angle .phi. on the network side of the respective converter. In FIG. 2, such a straight line is shown as an example for a load angle of .phi.=45.degree.. The operation of the circuit of FIG. 1 will be explained further by means of the transitions from an operating point AP1 to AP2 or from an operating point AP3 to AP4.
The operating point AP1 of a converter travels to AP2 if the active power reference value P* of the active power control R.sub.p is changed by the value .DELTA. P12. If in the circuit of FIG. 1 only the first cascade comprising the superimposed power controller Rp and the subordinated d-c current controller Ri were in operation, the operating point AP1, would adjust itself by changing the d-c current Id by the value .DELTA.Id12. If then, the second control cascade of the superimposed reactive power controller Rq and the superimposed quenching angle controller R1 were in operation, the undesired reactive power control deviation .DELTA. Q12 of the operating point API' from the desired operating point AP2 would be compensated by changing the quenching angle by the value .DELTA..gamma.12. Since normally both control cascades of the circuit of FIG. 1 are in operation, the transition between the operating points API and AP2 actually takes place on the locus curve OK12 shown in FIG. 2.
It can be seen from FIG. 2 that between the active power change .DELTA.P12 which represents the cause for the operating point transition, and the d-c current change caused by means of the control intervention of the subordinated controller Ri by the value .DELTA.Id12, an acute angle .delta.12 is present. For this reason, the second control cascade comprising the reactive power and the quenching angle control can compensate a reactive power control deviation caused by the control action of the d-c current control. This is done by adjusting the quenching angle by the value .DELTA..gamma.12 so fast that the operating point APl changes into the operating point AP2 on the locus curve OK12 without overshoot and with a small temporary reactive power control deviation.
The transition on the locus curve OK34, from the operating point AP3 to the operating point AP4 upon a change of the reactive power reference value by .DELTA.Q34, takes place in a very similar manner. Assuming that in this case, initially only the second control cascade of the superimposed reactive power controller Rq and the subordinated quenching angle control Rl is in operation, the operating point AP3 would first change to the operating point AP3, due to the control action of the controller Rq by a change of the quenching angle by the value .DELTA..gamma.34. If subsequently the first control cascade comprising the active power and the d-c current controllers were taken into operation, the undesired active power deviation .DELTA. P34 would be eliminated by a change of the d-c current by the value .DELTA. Id34. The operating point AP3, would thereby make the transition into the actually desired operating point AP4. Also in this case, the reactive point control deviation .DELTA. Q34, which represents the cause for the operating point transition, and the quenching angle change .DELTA..gamma.34 due to the control action of the subordinated controller Rq occupy an acute angle .delta.34 relative to each other. Thus, the transition from AP3 to AP4 takes place without overshoot in the normal case and with simultaneous operation with both cascade controls and with a small temporary active power control deviation on the locus curve OK34.
In this manner, any desired operating point transition due to a stepwise change, for example, of the active or reactive power reference value is possible with the known control of FIG. 1, in the portion of the family of characteristics shown in FIG. 2. This holds true for small and medium rectifier control angles or inverter quenching angles with good damping and therefore with high stability of all control loops.
It has been found, however, that the stability of such a control decreases with increasing rectifier control angle or inverter quenching angle and that a stability limit is reached at angle values between about 60.degree. and 70.degree.. In FIG. 2, a "straight stability line" SG is drawn as an example. This stability line or limit SG will be explained briefly by the example of the further portion, shown in FIG. 3, of a family of characteristics corresponding to FIG. 2. From this further portion can be seen specifically the operating points for a converter of an HVDC line operated as a rectifier or inverter which occur for medium and large rectifier control angles and inverter quenching angles.
In FIG. 3, the transitions from an operating point AP5 to AP6 and from an operating point AP7 to AP8 are shown by way of example. The operating point of a converter travels, for instance, from AP5 to AP6 if the active power reference value is changed by .DELTA. P56. It is assumed in this case that initially only the first cascade control of the active power and d-c current control Rp and Ri is in operation, so that the operating point AP5 is displaced due to the very large change of a d-c current actual value .DELTA. Id56 to the operating point AP5'. This operating point differs from the desired operating point AP6 by a large reactive power control deviation .DELTA. Q56. There is no longer an acute angle .delta. 56 between the change of the active power reference value .DELTA.P56 causing the operating point transition and the change of the d-c current due to the control action of the subordinated d-c current controller Ri in the region of the family of characteristics of FIG. 2. For this reason, the actual transition from the operating point AP5 to AP6 While both control cascades are in operation simultaneously, will take place with considerable overshoot on a locus curve OK56.
If such a transition behavior as above is present, it cannot be expected in practice that stable operating points can be set. Rather, due to the small damping in the control loops in this region of the family of characteristics, it is to be expected that a continuous oscillation of the actual operating point about the desired point occurs. This is because the very smallest disturbances on the line will cause a disproportionately large control action especially of the subordinated d-c current control Ri.
A transition, for instance, from the operating point AP7 along a locus curve OK78 to the operating point AP8 takes place in a very similar manner if the reactive power reference value is changed by .DELTA.Q78. Also in this case, the transition is accompanied by a considerable overshoot of the reactive s power control variable causing the change since an obtuse angle .DELTA..gamma.78 is present between the reactive power control deviation .DELTA.Q78 and the change in the quenching angle by .DELTA..gamma.78 caused by the subordinated quenching angle controller RL. This occurs if the second control cascade comprising the reactive power and the quenching angle control is operated alone. Practice has shown that with the known control structure of FIG. 1, no stable operating point can be adjusted above the straight stability line SG shown as an example in FIG. 3. Rather, all control variables hunt more or less violently and continuously about the desired reference values.